Itch, or pruritus, is a common and distressing symptom in a variety of diseases. Pruritus typically occurs in peripheral diseases such as allergic rhinitis, hemorrhoids, and dermatoses of fungal, allergic and non-allergic origin. Itching can also be a major symptom of certain systemic diseases such as Hodgkin's disease, chronic renal failure, polycythema vera, hyperthyroidism and cholestasis [see for example, Herndon, J. H., Jr., Int. J. Derm., 14, 456-484 (1975); Winkelman, R. K., Med. Clins. N. Am., 66, 1119-1133 (1982)]. The clinical importance of pruritus is undeniable but research efforts in this area have been modest, to a great extent owing to the absence of established, specific experimental models of the itch sensation/response, especially in preclinical research where animals that cannot communicate the nature of the sensation they are experiencing must be used.
The intracutaneous injection of histamine or proteases elicit itch, and this has been used as an experimental model for itch studies [Cormia, R. E., J. Invent. Derm. (Chicago), 76, 296-323 (1957)]. It was, therefore, postulated that these agents are involved as mediators in various itching conditions. However it has become apparent that central transmission of the itch sensation involves more than histamine or proteases. Since histamine was believed to be the primary mediator of the itch sensation, conventional itch therapy involves H.sub.1 -antihistamines as a first-line medication. However, antihistamines have no general anti-pruritic effect, in many instances they are either ineffective or only partially effective. The physician is often obliged to resort to glucocorticoids to relieve pruritus, but the potential undesirable side effects from glucocorticoid therapy are of great concern. Glucocorticoids cause skin atrophy and are absorbed systemically to cause Cushings disease-like effects. It has been concluded that although histamine is undoubtedly a potent pruritogen, there are other mechanisms operative in the clinically encountered spectrum of diseases where itch is a major symptom.
One important aspect in pruritus research is finding an accurate model of itch which allows reproducible determinations of pruritogenic compounds as well as compounds which alleviate itch. The itch sensation and nociception (perception of pain or injurious stimuli) had been believed to be somewhat similar in terms of neural transmission and central nervous system mediation. However a recent review article by Wahlgren, C.-F., Allergy, 47, pp. 65-75 (1992) on itching states:
Today's knowledge of the pathophysiology of itching is based on studies of human subjects, as no research models for the investigation of itching in animals are established. Although scratching often occurs in animals, there is no definite proof that it always reflects itching. The highly subjective nature of pruritus makes it an intricate task for the investigator to evaluate. To circumvent the problem of measuring such a subjective symptom, some investigators have developed methods for measurement of scratching, as this is the objective correlate to itching. PA1 Itching can be elicited by chemical, electrical, mechanical and thermal stimulation. No morphological structure has been identified as a specific receptor for the itch sensation, but it is assumed that itch receptors are linked to the free nerve endings of C-fibers close to the dermo-epidermal junction. The impulses set up in the thin, non-myelinated, slowly conducting C-fibers enter the spinal cord via the dorsal horn, then ascend in the contralateral spinothalmic tract, pass via the thalamus and end in the somatosensory cortex of the post-central gyrus. Itching and pain are related phenomena, and it was previously believed that itching was equal to sub-threshold pain, i.e. with increased activity in the C-fibers the perceived sensation changed from itching to pain. Today, itching and pain are considered as two separate sensory modalities. The reasons for this are: a) that both sensations can be felt simultaneously, b) that if the epidermis is removed itching cannot be elicited, whereas pain can still be provoked, c) that itching and pain evoke different motor responses (scratching and withdrawal, respectively), d) that opioids provoke itching, but relieve pain, and e) recent neurophysiological findings. By inserting micro-electrodes percutaneously in peripheral C-fibers of conscious volunteers, Torebjork & Ochoa [Soc. Neurosci. Abstr., 7 p.228 (1981)] showed that electrical stimulation of some C-fibers provoked itching, whereas stimulation of other C-fibers induced pain. If itching was induced, increased stimulation frequency gave an increased itch intensity, but itching never changed to pain. Further studies, where pruritus was provoked by electrical stimulation or histamine iontophoresis, have confirmed this. PA1 The central mechanisms involved in the modulation, interpretation and processing of pruritus are completely unknown, but undoubtedly the central nervous system has a great influence on the perception of itching. For example, emotional stress or anxiety can aggravate pruritus, whereas distraction can relieve it. [italics added] PA1 5-HT and 5-HT receptors appeared early in evolution before the separation of vertebrates and invertebrates. This may be related to the fact that in most species 5-HT is involved in the modulation of neuronal activity, a feature that is likely to be required even in primitive organisms in order to adapt to changes in the environment. PA1 Most 5-HT receptors belong to the G-protein coupled family of receptors, the specificity of which involves several parameters: affinity for 5-HT, coupling with different second messenger systems, expression patterns and subcellular localization. The expression pattern of these receptors is often complex and not limited to a particular organ, suggesting that their role is not confined to a particular physiological function but rather to modulation of multiple circuits in order to influence various components of complex behaviors such as aggression or escape. PA1 Functional and receptor binding studies of the actions of 5-HT receptor agonists and antagonists have led to claims of no less than eleven subtypes of 5-HT receptor. Of these, five have been positively differentiated by receptor cloning and purification. Attempts to identify receptor selective drugs have so far yielded selective agonists for 5-HT.sub.1A, 5-HT.sub.1B, and 5-HTS.sub.3 and selective antagonists for 5-HT.sub.2 and 5-HT.sub.3 receptors. Clearly 5-HT receptor pharmacology is in its infancy.
Thus the state of the art has been shifted to separate nonciceptive neural stimulation a part from transmission of the pruritic sensation.
A number of articles have published in the scientific literature on the scratching behavior induced by substance P (a decapeptide, also known as Neurokinin P) by intrathecal (spinal) or intracerebral administration. It is now generally acknowledged that the reciprocal hind limb scratching directed towards the sides of the upper body and the jaw areas after Substance P (or other stimulatory agent) injection is a response to perceived pain, or is alternately an efferent motor response to receptor-based stimulation. Some of the scratching responses are described as non-purposeful or stereotyped movements, indicating that they are not necessarily in response to any perceived sensation at all, either nociceptive or pruritic in origin, but rather result from stimulation of motorneurones that innervate the hind limb muscles. Licking and biting behavior is also manifested with intrathecal or intracerebral injection of substance P. See, for example, Hylden, J. L. K., et al., Brain Research, 217, pp. 212-215 (1981), Piercey, M. F., Brain Research, 210, pp. 407-412 (1981), Hylden, J. L. K. and Wilcox, G. L., Journal of Pharmacology and Experimental Therapeutics, 226(2), pp. 398-404, (1983), and Meisenberg, G. and Simmons, W. H., Peptides, 7, pp. 557-561 (1986).
In the present invention, the inventor has taken great pains to significantly link animal scratching behavior with a clinically known syndrome of itch, i.e. experimental allergic conjunctivitis. The establishment of this animal model of clinically encountered pruritus has also been disclosed in co-pending U.S. application Ser. No. 07/837,568, commonly assigned by the same inventor, and is incorporated herein by reference in its entirety. A number of independent lines of evidence indicate that the itch-scratch response (ISR) to substances administered to the ocular surface reflects a behavioral response to the itch sensation. These can be enumerated as follows: 1) itch-scratch can be elicited by topical administration of antigen to the eyes of presensitized guinea pigs, closely mimicking the situation encountered in allergic conjunctivitis patients; 2) itch-scratch responses occur as part of a typical response to allergic provocation that includes redness, swelling, and leukocyte infiltration into the conjunctiva, which again closely mimics the situation encountered in allergic conjunctivitis patients; 3) histamine, a well-characterized benchmark pruritogen, potently elicits itch-scratch episodes directed to the conjunctiva; 4) drugs known to be of benefit in treating allergic conjunctivitis patients, such as pyrilamine and ketorolac, reduced the ocular itch-scratch episodes associated with allergic provocation in this model; 5) the scratching is entirely and specifically directed towards the afflicted area, i.e. the conjunctiva; 6) the animals appear to exert conscious control over itch-scratch responses, which are interrupted by external sensory stimuli; 7) painful stimuli and foreign bodies do not elicit itch-scratch responses.
The pharmacology of systemic 5-HT analog effects on the itch-scratch responses associated with experimental allergic conjunctivitis is very different from that associated with intrathecal substance P induced hind limb scratching. See, for example: Alhaider, A. A., et al., Eur. J. Pharmacol., 249 (2), pp. 151-160 (1993). All 5-HT ligands, either agonists or antagonists, tested in the reduction of the present invention to practice inhibited the itch-scratch response associated with experimental conjunctival allergy. In Alhaider, intrathecal 5-methoxy-N,N-dimethyl tryptamine (5-MeO-DMT), a 5-HT.sub.1A /5-HT.sub.1B receptor agonist enhanced intrathecal substance P induced scratching. This is in complete contrast to the inhibitory effect achieved with 5-HT.sub.1A and 5-HT.sub.1B agonists (8-OH-DPAT, CGS 12066B) in the allergic conjunctival itching model. Furthermore, 5-MeO-DMT enhancement of intrathecal substance P induced scratching was inhibited by MDL 72222 and spirotraxine, but not ketanserin. This differs from the itch-scratch response associated with experimental allergic conjunctivitis in that ketanserin is also inhibitory in this model. These findings further support the literature in distinguishing nociception from pruritus.
Another study by Berendsen and Broekkamp [Berendsen, H. H. G. and Broekkamp, C. L. E., Eur. J. Pharmacol., 194, pp. 201-208 (1991) sought to identify the pharmacological characteristics of hind limb scratching induced by serotonergic compounds. The authors hypothesized that 5-MeO-DMT (and other 5-HT analogs as well as 5-HT itself) induced scratching in the rat after subcutaneous injection which was mediated by serotonergic receptors. The authors concluded that the hind limb scratching induced by 5-HT analogs is probably mediated by binding to the 5-HT.sub.1D receptor outside the blood-brain barrier. Other 5-HT ligands were found to potentiate the hind limb scratching induced by 5-MeO-DMT, in particular they note that the 5-HT.sub.1D agonist methiothepin had this effect. Other 5-HT ligands were found to attenuate scratching behavior, and it was also noted that .alpha..sub.2 antagonists could also attenuate hind limb scratching. Nowhere in the article do the authors state or infer that hind-limb scratching elicited in this study is symptomatic or indicative of itch (or pruritus).
5-Hydroxytryptamine (5-HT), also called serotonin, is a neurotransmitter present in most organisms ranging from humans to species with quite primitive nervous systems. 5-HT elicits a wide variety of behaviors such as aggressive postures in lobsters, feeding and learning in snails, locomotion in lampreys and affects sleep and appetite in mammals. This appearance across species and even across tissues within species can be explained both by the wide distribution of 5-HT and the multiplicity of 5-HT receptors.
Several distinct 5-hydroxytryptamine (5-HT) subtypes, grouped in four main classes of 5-HT.sub.1, 5-HT.sub.2, 5-HT.sub.3, and 5-HT.sub.4 receptors, are known to mediate the physiological effects of the neurotransmitter. Since the late 1970's, binding studies have led the way in the initial identification of distinct 5-HT.sub.1 and 5-HT.sub.2 receptors, and the subsequent subdivision of 5-HT.sub.1 receptors into 5-HT.sub.1A, 5-HT.sub.1B, 5-HT.sub.1C (recently reclassified as 5-HT.sub.2C), 5-HT.sub.1D, 5-HT.sub.1E binding sites. For a discussion of receptor subtypes and classification see Humphrey, P. P. A., et al., TiPS, 14, (June) pp. 233-236 (1993).
Examples of 5-HT receptor agents that have found clinical or pharmacological use are: buspirone, a 5-HT.sub.1A partial agonist that is used as an anxiolytic; ketanserin, a 5-HT.sub.2 antagonist (also at .alpha..sub.1 receptors) is used as an antihypertensive agent; quipazine, a 5-HT.sub.3 agonist has found use as an antidepressant; and sumatriptan a 5-HT.sub.1D agonist is used in treating migraine headaches. ##STR1##
For a review of 5-HT receptor agonists and antagonists and associated biochemical and pharmacological aspects see Middlemiss, D. N. and Tricklebank, M. D., Neurosciences and Biobehavioral Reviews, 16, pp. 75-82 (1992).
Another review article on 5-HT receptors by R. Hen [TiPS, 13, (April) pp.160-165 (1992) concludes:
The review by Middlemiss and Tricklebank (vide supra) concludes: